Glass fiber reinforced laminates prepared from cured epoxy resins are well known in the art. The use of difunctional brominated epoxy resins produces laminates having self-extinguishing properties. See, for example, Lapitz, Jr., U.S. Pat. No. 3,600,263 and Fujiwara et al, U.S. Pat. No. 3,741,858, the disclosures of which are incorporated herein by reference. A disadvantage noted is the tendency of such laminates to be swelled or attacked by solvents during fabrication of printed circuits. Solvent resistance has been improved by mixing the difunctional brominated epoxys with tetrafunctional epoxys prior to impregnation of the glass fibers; see Chellis, U.S. Pat. No. 3,523,037, the disclosure of which is incorporated herein by reference. In his mixed epoxy laminating systems Chellis uses a curing system comprising dicyandiamide catalyzed with a tertiary amine. It has been found that such a curing system suffers the disadvantage of producing B-staged saturated sheets (prepregs) which are not particularly consistent within a lot or from lot-to-lot. The prepregs produce laminates of the single and multilayer type which are not as consistent and reproducible in their properties as would be desired Lopez et al, U.S. Pat. No. 3,391,113, the disclosure of which is incorporated herein by reference, disclose that di-functional epoxy resin compositions can be rapidly cured at lower than expected temperatures with a curing system comprising curing amounts of dicyandiamide and from 0.01 to 1 part by weight of dicyandiamide of a tetramethyl- or tetraethyl-guanidine. The system is stated in Col. 2, lines 15-16, to have a relatively low shelf life. Lopex et al. have to overcome this and extended shelf life by adding acids such as benzoic acid in substantial quantities.
It has now been discovered that superior laminating compositions and prepregs can be made by mixing di- and tetra-functional epoxides and curing them with a system comprising dicyandiamide and a tetraalkylguanidine. Unexpectedly, there is no need to introduce stabilizing additives to overcome the expected low shelf life because stable prepregs are produced in spite of the teachings in Lopez et al, when instead of difunctional epoxy resins, a mixture of di- and tetra-functional epoxy resins is used. In addition, it is preferred to use a difunctional epoxy resin with a lower molecular weight than that employed by Chellis thereby obtaining more consistent and more reproducible prepreg and bonding sheet materials. The present invention uses one with an epoxy equivalent weight (EEW) of 370-430, whereas Chellis states that an EEW or 455 to 500 is critical.